A magnetic recording read channel converts an analog read signal into an estimate of the user data that was recorded on a magnetic medium. Read heads and magnetic media introduce noise and other distortions into the read signal. For example, as the information densities in magnetic recording increase, the intersymbol interference (ISI) becomes more severe (i.e., the channel impulse response becomes longer). ISI is a form of signal distortion in which one symbol interferes with one or more other symbols.
In a conventional read channel, a continuous-time filter (CTF) typically processes the read signal in the analog domain to perform anti-alias filtering, band-limit filtering to reduce electronic noise, and signal shape filtering to reduce ISI. Generally, anti-alias filtering removes noise and residual signal components above the Nyquist frequency (equal to half the baud rate frequency) to avoid aliasing. An analog-to-digital converter (ADC) typically processes the CTF output to generate digital samples for further processing in the digital domain. A Viterbi detector is often used in a read channel to process the digital samples and detect the recorded data bits in the presence of intersymbol interference and other noise.
As process technology gets smaller and data rates increase, it becomes increasingly challenging to build analog circuits, such as the CTF filters, that meet the demanding performance specifications of read channels. A need therefore exists for improved read channels that transfer a portion of the signal processing burden from the analog domain to the digital domain, to thereby simplify the analog circuitry design. A further need exists to improve the signal-to-noise ratio (SNR) and error rate performance of read channel devices. A need therefore exists for improved read channels that allow more complex signal processing techniques to be applied in the digital domain. Yet another need exists for improved read channels that provide selective oversampling of the analog to digital conversion.